Zusammenfassung

The CNS consists of a large number of neurons organized in different brain regions and connected into a complex network. Studies of the connectivity of the brain has emphasized the point to point direct connections between brain regions. However, it has often been argued that in principle only a few synaptic steps are required to propagate activity from any location in the brain to any other. Hence not surprisingly questions regarding the efficacy, gating properties and coincident activation have become important issues as to how activity is propagated polysynaptically. The advent of the method of esfMRI - combining electrical stimulation with fMRI - has provided us with a new method to study the connectivity in spatially distributed networks. Here we show that electrical stimulation of the deep cerebellar nuclei (DCN) reveals BOLD responses in different brain sites that are only indirectly connected to the stimulation site, in fact we observe BOLD responses in brain sites that are dislodged away by at least three synaptic steps from the DCN. Hence we show that some neuronal pathways can propagate synchronous stimuli effectively and can lead to the activity of widespread brain regions via polysynaptic pathways that have not been considered so far. These findings are in marked contrast to our previous observations of a lack of propagation of electrically induced activation when stimulating neocortical brain sites (Tolias et al. 2005). Hence, our findings point to surprisingly divergent behaviours in different networks in their ability to propagate synchronous activity. We speculate that under physiological conditions these excitable subcortical networks are controlled by the inhibition of the cerebellar cortex which in our experiments is short-circuited by electrical stimulating the DCN.